Microphones have been known that are used under high acoustic pressure near musical instruments to acquire sounds of the instruments. Condenser microphones are commonly employed for use near instruments, which can be more easily downsized than dynamic microphones so as not to obstruct the performer's action or view. Sounds of instruments acquired by the microphone are often subjected to amplification (also referred to as “PA (Public Address)”) at a concert hall, for example. Such sounds have unidirectionality in general. A unidirectional condenser microphone includes a microphone case having openings on its side wall, the openings communicating with a rear acoustic terminal provided at a condenser microphone unit. The openings introduce sound waves to the rear acoustic terminal of the condenser microphone unit.
A condenser microphone requires an electronic circuit such as an impedance converter for reducing the output impedance of the microphone unit. In this respect, the condenser microphone differs from a dynamic microphone outputting signals from the both ends of a voice coil. The condenser microphone requires an electronic circuit as described above, and should be as small as possible. The condenser microphone therefore includes a circuit board having an electronic circuit thereon near the rear acoustic terminal of the microphone unit.
When such a condenser microphone is used near instruments, for example, sound waves with high acoustic pressure entering the microphone case cause vibration of the circuit board. The vibration of the electronic circuit mounted on the circuit board induces incidental noise other than intended sounds to be electro-acoustically converted by the microphone unit. Such incidental noise results in deterioration in sound quality. Thus, a solution to prevent the electronic circuit from vibrating in response to sound waves has been desired.
The microphone case has an internal space communicating with the openings of the microphone case and located behind the rear acoustic terminal of the microphone unit. The space accommodates the circuit board having the electronic circuit. Since the space serves as acoustic capacitance, the acoustic mass of the space and the openings of the microphone case generate resonance, which deteriorates the frequency response of the microphone. If the space has a large volume, the space could easily accommodate the circuit board and the electronic circuit elements thereof; however, the lowered impedance of the acoustic capacitance makes resonance more likely to occur in the main frequency range of acquired sounds.
A reduction in volume of the space should therefore increase the impedance of the acoustic capacitance. The reduced volume of the space leads to an increase in the impedance of the acoustic capacitance, thereby increasing the resonance frequency of the space. Thus, for a resonance frequency of the space higher than the upper limit of the main frequency range of acquired sounds, the directional frequency response does not deteriorate in a frequency range equal to or lower than the resonance frequency.
A known condenser microphone includes a space behind a rear acoustic terminal of a condenser microphone unit inside a microphone case, and further includes a filler functioning as acoustic resistance in the space to prevent the resonance of the space. In the condenser microphone disclosed in Japanese Unexamined Patent Application Publication No. 2011-9807, a filler is a sintered plastic material having countless continuous pores and disposed in the space inside the microphone case. The filler functions as acoustic resistance against sound waves entering the space through openings on the side wall of the microphone case, and prevents the resonance of the space.
The invention according to Japanese Unexamined Patent Application Publication No. 2011-9807 certainly prevents the resonance of the space inside the microphone case. However, the invention according to Japanese Unexamined Patent Application Publication No. 2011-9807 does not intend to include an electronic circuit in such a small space inside the microphone case, and also not intend to prevent the electronic circuit from vibrating in response to sound waves entering the microphone case.
An object of the present invention is to provide a condenser microphone that includes a space inside a microphone case for accommodating an electronic circuit with a reduced volume to prevent the resonance of the space, and can prevent the electronic circuit from vibrating in response to sound waves.